Technical Field
The present disclosure relates to an offset voltage generator and a method for generating an offset voltage for controlling continuous modulation and discontinuous modulation of a three-phase inverter.
Description of the Related Art
As power semiconductor technology evolves, it has become easier to implement variable voltage and variable frequency (VVVF) driver by using power devices that can be switched at high speed. A voltage source inverter that typically receives DC voltage source to generate AC variable voltage source is commonly employed as a circuit for generating variable voltage variable frequency. Such a voltage source inverter is commonly employed by energy storage system (ESS), PV inverter, motor drive technologies.
In modulating voltage generated by such a voltage source inverter, a variety of types of modulation schemes may be employed. One of the most commonly used modulation scheme is pulse width modulation (PWM) scheme. The PWM scheme may be divided into continuous modulation scheme and discontinuous modulation scheme. Examples of the continuous modulation mode may include sinusoidal PWM (SPWM), space vector PWM (SVPWM), etc. Examples of the discontinuous modulation mode may include 60° discontinuous PWM (DPWM) for reducing switching loss of a power semiconductor.
In such continuous or discontinuous modulation modes, an offset voltage is compared to a carrier wave such as a triangular wave to modulate voltage. In doing so, different offset voltages are used in the continuous and discontinuous modulation modes. In the related art, there is a problem in that when a three-phase inverter is switched from the continuous modulation mode to the discontinuous modulation mode, an offset voltage for the discontinuous modulation mode has to be calculated according to a modulation index ML In addition, according to existing voltage modulation scheme using offset voltage, offset voltage is continuously introduced even in a period where the offset voltage is not necessary, and thus zero sequence voltage always exists in the inverter.